METASTABLE PYROXENES AND THEIR ROLE IN THE SUBDUCTION PROCESS

Abstract

Pyroxene is an important group of minerals in the subducted slab and the upper mantle. In the upper mantle, where the temperature is high, pyroxene gradually dissolves into garnet and forms majoritic garnet at the base of the upper mantle and the transition zone. However, recent studies show that the pyroxene to garnet transition is very slow, especially in the cold subduction zones. Therefore, pyroxene is expected to be preserved metastable to the transition zone or even the top of the lower mantle in the cold subducted slabs. In this thesis, the high pressure behavior of two clinopyroxenes — hedenbergite and diopside are studied at high pressure conditions. The compressional behavior of hedenbergite (CaFeSi2O6 ), end-member diopside and iron-bearing diopside are studied by single crystal X-ray diffraction. The unit cell parameters and equation of state are reported and compared. At the upper mantle depths, eclogite with aluminous iron-bearing diopside is denser than eclogite with end-member diopside in the cold subducted slab conditions, and therefore provides larger slab pulling force. At the bottom of the transition zone and the top of the lower mantle, eclogite with aluminous iron-bearing diopside, though has higher density than that with end-member diopside, is still less dense than the surrounding mantle and could contribute to slab stagnation — a failure of the slab to descend into the lower mantle. A new phase (γ-diopside) with Si in a rare five-fold coordination site is observed above 50 GPa. This result suggests that V Si may exist in the transition zone and the uppermost lower mantle in appreciable quantities and may have significant influences on buoyancy, wave velocity anomalies, deformation mechanisms, chemical reactivity of silicate rocks and seismicity within the slab. The γ-diopside is ∼ 6% and ∼ 11% lighter than MgSiO3 akimotoite and CaSiO3 + MgSiO3 perovskite mixture, which would promote stagnation of the cold slab in the transition zone or the uppermost part of the lower mantle. All studies suggests that metastable pyroxenes, even after phase transitions, are less dense than the surrounding mantle minerals in the transition zone or the top of the lower mantle, and can contribute to slab stagnation.